azymik, it is to do with the inertia of the aeroplane. The aeroplane, being a heavy mass, wants to continue with its same groundspeed. When you start getting affected by a tailwind, imagine a tailwind as fast as the aeroplane's groundspeed, the aeroplane will suddenly have a zero airspeed and will drop out of the sky! An increasing tailwind makes the aeroplane lose airspeed and therefore lift.
No speed restriction, I think you have that the wrong way round. You will descend at an earlier point to benefit from the tailwind in a glide descent. What is the point in maintaining altitude? You will then have to descend with speedbrakes out. In a headwind, you have to cruise longer and descend at a later point to prevent you having to power up in the descent to stop descending far too early- again, why? The aim of the game is to descend so as to arrive at destination with idle power throughout. Therefore, if your descent takes 30 minutes to get down from cruise altitude, the most efficient descent point is where you will be 30 minutes from destination at idle power- further in a tailwind and less far in a headwind.
You are an airline pilot and you have a serious misconception like that, totally confusing our foreign language contributors?